Precipitation driven patterns in soil biogeochemistry throughout the soil profile across 20kyr old basaltic soil profiles

Background/Question/Methods

Precipitation drives nonlinear and irreversible changes in soil biogeochemistry, as is demonstrated by previous work focused on significantly older soils.  Here, we asked how soil biogeochemical properties throughout the soil profile respond to variations in precipitation, when the precipitation regime has been enforced on timescales of 20kyr. We sampled soils from across a rainfall gradient that ranged from 500 to 5500mm of annual rainfall. The soils are developing on a basaltic substrate that is roughly 20kyr old, on the windward side of Mauna Kea Volcano, HI. Sampling included 8 complete soil profiles (ranging in depth between 50cm and 150cm), and 9 parent material samples. Samples were analyzed for bulk chemistry, exchangeable cations, cation exchange capacity, base saturation, and nutrient availability.  Percent of element remaining in the soil relative to parent material was calculated using niobium as an immobile reference element. 

Results/Conclusions

In sites receiving <1000mm of rainfall, remaining base cations (Ca_r, Mg_r, K_r, and Na_r) are either unchanged with depth, or indicate translocation to depth. In sites receiving >2000mm of rainfall, >80% of each base cation, except for Mg, has been lost from throughout the soil profile. In sites between 1000 and 2000mm of rainfall, there is evidence of surface enrichment, with Mg_r> Ca_r> Na_r> K_r throughout the soil profiles. However, in this intermediate region of rainfall the concentrations of exchangeable cations are low (0 - 20mEq/100g), and [Ca²⁺]> [Mg²⁺]> [K⁺]> [Na⁺] throughout the soil profile. These results indicate that while Mg²⁺ is less likely than Ca²⁺ to be weathered from solid materials, Ca²⁺ is more strongly retained as exchangeable ions upon release. All soils analyzed had pH’s between 3.5 and 6.5, and soil pH decreases as rainfall increases. Above ~2000mm of rainfall, within each soil pit the surface horizons are approximately 1 pH unit lower than deeper horizons. These patterns indicate changes in the buffering system, with a transition from soil pH being controlled by cation weathering and exchange reactions in drier sites, to being controlled by Al dissolution and hydrolysis reactions in wetter sites.